You can composite different OBJ files together within the CR2 file.

The objFileGeom must reference complete files, you can not call individual groups in the way that the figureResFile does. This would mean that you would need to first export out the M3 body parts and save them to a designated folder before loading this figure.

That could conceivably be executed with a read script.

{

version
 {
 number 4
 }
runPythonScript ":Runtime:Python:poserScripts:scriptToLoadM3ThenExportRequiredBodyParts.py"
figureResFile :Runtime:Geometries:P4NudeMan:blNudeMan_Gen.obj
actor BODY:2
 {

 }
actor hip:2
 {
 storageOffset 0 0 0
 geomHandlerGeom 13 hip
 }
actor abdomen:2
 {
 storageOffset 0 0 0
 geomHandlerGeom 13 abdomen
 }
actor chest:2
 {
 storageOffset 0 0 0
 geomHandlerGeom 13 chest
 }
actor neck:2
 {
 storageOffset 0 0.3487 0
 objFileGeom 0 0 :Runtime:Geometries:Your Folder:custom neck.obj
 }
actor head:2
 {
 storageOffset 0 0.3487 0
 objFileGeom 0 0 :Runtime:Geometries:Your Folder:custom head.obj
 }
actor leftEye:2
 {
 storageOffset 0 0 0
 geomHandlerGeom 13 leftEye
 }